Journal of the Geological Society, London, Vol. 163, 2006, pp. 671–682. Printed in Great Britain.
Structure and evolution of hydrothermal vent complexes in the Karoo Basin,
South Africa
1
H E N R I K S V E N S E N 1 , B J Ø R N JA M T V E I T 1, S V E R R E P L A N K E 1,2 & L U C C H E VA L L I E R 3
Physics of Geological Processes (PGP), Department of Physics, PO Box 1048, Blindern, University of Oslo, Norway
(e-mail: hensven@fys.uio.no)
2
Volcanic Basin Petroleum Research (VBPR), Oslo Research Park, 0349 Oslo, Norway
3
Council for Geoscience, PO Box 572, Bellville 7535, Cape Town, South Africa
Abstract: The Karoo large igneous province, formed at c. 183 Ma, is characterized by the presence of
voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal
vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the
horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent
complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude
tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders.
Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal
mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one
or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in
which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating
and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The
hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic
aureoles, slightly predating the onset of the main phase of flood volcanism.
have provided constraints on the spatial relationship between sill
intrusions and hydrothermal vent complexes (Planke et al. 2005).
Generally, the hydrothermal vent complexes represent conduit
zones up to 8 km long rooted in contact aureoles around sill
intrusions, where the upper part of the vent complexes comprise
eyes, craters or mounds, up to 10 km in diameter (Fig. 1; Planke
et al. 2005). Field observations from the Karoo Basin show that
similar hydrothermal vent complexes commonly comprise breccias of sedimentary rocks with a varying degree of magmatic
material (e.g. Dingle et al. 1983; Woodford et al. 2001; Jamtveit
et al. 2004).
The aim of this paper is to document the structure, composition and evolution of sediment-dominated hydrothermal vent
complexes in the Karoo Basin, South Africa (Fig. 2a). Here, we
have chosen to focus on two representative complexes that were
selected after reconnaissance fieldwork on more than 10 complexes in the Dordrecht–Rossow area in the Eastern Cape
Province (Fig. 2b). Specifically, we want to address the following
questions that can be tested by detailed fieldwork and petrographic analysis. (1) Do sediment-dominated hydrothermal vent
complexes represent phreatic explosion events? (2) What is the
involvement of igneous material in phreatic sediment-dominated
vent complexes? (3) What is the degree of hydrothermal alteration in and around short-lived phreatic systems? Quantitative
aspects of the formation of hydrothermal vent complexes and
processes in volcanic basins are addressed elsewhere (Jamtveit et
al. 2004; Malthe-Sørenssen et al. 2004).
Large igneous provinces, such as the North Atlantic and the
Karoo–Lesotho provinces, are characterized by the presence of
an extensive network of sills and dykes emplaced in sedimentary
strata (e.g. Du Toit 1920; Walker & Poldervaart 1949; Chevallier
& Woodford 1999; Berndt et al. 2000; Brekke 2000; Bell &
Butcher 2002; Smallwood & Maresh 2002; Trude et al. 2003;
Planke et al. 2005). An important consequence of intrusive
activity in sedimentary basins is that the magma causes rapid
heating of the intruded sediments and their pore fluids, causing
expansion and boiling of the pore fluid (Jamtveit et al. 2004),
and metamorphic dehydration reactions. These processes may
lead to phreatic volcanic activity by the formation of cylindrical
conduits that pierce sedimentary strata all the way to the surface.
The hydrothermal vent complexes thus represent pathways for
gases produced in contact aureoles to the atmosphere, with the
potential to induce global climate changes (Svensen et al. 2004).
Consequently, constraints on processes leading to the formation
of hydrothermal vent complexes in sedimentary basins, their
abundance and structure may lead to a better understanding of
the causes of the abrupt climate changes that are associated with
many large igneous provinces (e.g. Wignall 2001; Courtillot &
Renne 2003).
Hydrothermal and phreatomagmatic vent complexes are recognized from several sedimentary basins associated with large
igneous provinces, including the Vøring and Møre basins off
mid-Norway (Skogseid et al. 1992; Svensen et al. 2003; Planke
et al. 2005), the Faeroe–Shetland Basin (e.g. Bell & Butcher
2002), the Karoo Basin in southern Africa (e.g. Du Toit 1904,
1912; Gevers 1928; Stockley 1947; Dingle et al. 1983; Jamtveit
et al. 2004), in the Karoo-equivalent basins of Antarctica (Grapes
et al. 1973; Hanson & Elliot 1996; White & McClintock 2001),
and the Tunguska Basin in Siberia, Russia (e.g. Zolotukhin &
Al’mukhamedov 1988). Two- and three-dimensional seismic data
Terminology
Volcanic basins are sedimentary basins with a significant amount
of primary volcanic rocks (e.g. sills and dykes).
Pierced basins are sedimentary basins with many piercement
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on
ne
zo
n
In
er
ut
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H. SVENSEN ET AL.
e
672
Crater and
surface deposits
Upper
part
Conduit
Inward dipping
strata
Lower
part
Fluids
(liquid/gas)
Heating/boiling
Sill
Contact aureole
and aeolian (upper part of the Stormberg Group) (Catuneanu et
al. 1998). At the time of flood basalt eruption, up to 400 m of
fine-grained sand had been deposited by aeolian, fluvial and
lacustrine processes (Smith 1990; Veevers et al. 1994). The
sedimentary rocks forming the southwestern parts of the basin
were gently folded during the Cape Orogeny (278–215 Ma;
Catuneanu et al. 1998), whereas the rest of the basin is
essentially undeformed.
Both southern Africa and Antarctica experienced extensive
volcanic activity in the early Jurassic Period. Dolerites and lavas
in the Karoo Basin were emplaced and erupted within a
relatively short time span (183 1 Ma; Duncan et al. 1997). Sills
and dykes are present throughout the sedimentary succession,
and locally form up to 70% of the basin volume (e.g. Rowsell &
De Swardt 1976). Hydrothermal vent complexes and volcanic
phreatic complexes are numerous in the Karoo Basin, and
primarily exposed within the Stormberg Group sediments. The
complexes comprise a range of different rock types, ranging
from lava and pyroclastic rocks to sediment breccia and sandstone (e.g. Du Toit 1904, 1912; Gevers 1928; Taylor 1970;
Dingle et al. 1983). The formation of the complexes is related to
the sill emplacement episode and the formation of the Karoo
igneous province (e.g. Dingle et al. 1983; Woodford et al. 2001;
Jamtveit et al. 2004). The hydrothermal vent complexes represent a very prominent feature of the Karoo Basin, and more than
320 have been identified in the Stormberg Group.
Methods
~300 m
Fig. 1. Idealized cross-section through a hydrothermal vent complex
based on field observations from the Karoo Basin and seismic
interpretation from the Norwegian Sea (Jamtveit et al. 2004; Planke et al.
2005). We divide hydrothermal vent complexes into a lower and an upper
part, and an inner and an outer zone.
structures such as mud volcanoes, dewatering pipes and hydrothermal vent complexes.
Sills are tabular igneous intrusions that are dominantly layer
parallel. They are commonly subhorizontal. Sills may locally
have transgressive segments, i.e. segments that cross-cut the
stratigraphy.
Hydrothermal vent complexes are pipe-like structures formed
by fracturing, transport and eruption of hydrothermal fluids. The
complexes described here are dominated by sedimentary rocks
with a negligible content of igneous material.
Sediment volcanism is surface eruption of mud, sand or
sediment breccias through a vent complex.
The Karoo Basin
The Karoo Basin (Fig. 2a) covers more than half of South Africa.
The basin is bounded by the Cape Fold Belt along its southern
margins and comprises up to 6 km of clastic sedimentary strata
capped by at least 1.4 km of basaltic lava (Fig. 2c; Smith 1990;
Johnson et al. 1997). The sediments were deposited from the
Late Carboniferous to the Mid-Jurassic, in an environment
ranging from partly marine (the Dwyka and Ecca groups), to
fluvial (the Beaufort Group and parts of the Stormberg Group),
Detailed mapping of two hydrothermal vent complexes, Witkop II and III
(Fig. 2b), was carried out during three field seasons, including an
extensive sampling programme (c. 100 samples). The fieldwork focused
on mapping the total extent of the vent complexes, identifying and
mapping sedimentary facies units and their boundaries, logging of
sections in the vent complexes, and identifying structural features such as
faults and pipes. Aerial photographs from the Chief Directorate of
Surveys and Mapping in Cape Town (South Africa) were used for
geological mapping because of the lack of detailed topographic sheets for
the area. Length and height measurements were made by a Garmin Etrex
global positioning system (GPS), and used to calibrate maps and logs.
Petrography of a large number of thin sections (.50) has been studied
by optical and electronic microscopes at the Department of Earth
Sciences, University of Oslo, with a JEOL JSM 840 SEM and a
CAMECA SX 100 electron microprobe. A microprobe analysis of zeolite
was carried out with an accelerating voltage of 15 kV, a beam current of
5 nA and a 5 ìm raster size, using synthetic and natural standards.
The Department of Water Affairs in South Africa drilled a percussion
borehole trough the inner zone of the Witkop III hydrothermal vent
complex in April 2003. The borehole started at 1908 m above sea level,
and terminated at 300 m below the surface. Chips were collected every
metre during drilling, and were washed and logged.
Facies units
Geological mapping of the two hydrothermal vent complexes
revealed the presence of distinct sediment facies units (Fig. 3).
These include (1) sandstone in the inner zone of the vent
complex, (2) zeolite-cemented sandstone, and (3) sediment
breccia. Descriptions of the surrounding Elliot and Clarens
formations have been given by Catuneanu et al. (1998).
Homogeneous sandstone in the inner zone of the vent
complex (Unit I)
Figure 3b shows typical sandstone from Unit I, with a dominance
of quartz and feldspar grains. The vent complex sandstone
SOUTH AFRICA
Witkop III
Lesotho
Cape
Town
FOLD
18 E
Karoo
Sediments
BELT
22 E
26 E
Flood Basalts/
Lebombo volcanics
O
CE
AN
THE KAROO BASIN
IN
DI
AN
Study area
PE
CA
IC
NT
LA N
AT CEA
O
34 S
31 13’
Durban
30 S
400 km
Breccia Pipes
HTVC
31 15’
Witkop II
27 14’
27 08’
Clarens Fm.
Drakensberg Group
Elliot Fm.
Hydrothermal vent complex
Molteno Fm.
Dolerite sill
Dolerite dyke
Ss
Ss
shl
Ss
shl
Ss
shl
C
HTVC
Johannesburg
26 S
VVV
VVV
VVV
0m
Molteno Elliot Clarens
B
27 14’
2 km
Permian
Jurassic
Triassic
Dwyka Ecca Beaufort
Stormberg
A
BOTSWANA
NAMIBIA
673
Fm.
Sills
H Y D ROT H E R M A L V E N T S I N T H E K A RO O BA S I N
700
?
3100
Shl
4100
4400
V Drakensberg Group lava
Dwyka Group shale/tillite
HTVC:
Hydrothermal vent
complex
Fig. 2. (a) Distribution of hydrothermal vent complexes (HTVC) and breccia pipes in the Karoo Basin, South Africa. The hydrothermal vent complexes
are mainly confined to outcrops in the Stormberg Group sediments, whereas the breccia pipes are confined to the Ecca and Beaufort groups (Woodford et
al. 2001). Fieldwork has been carried out on more than 10 hydrothermal vent complexes in the study area (the Molteno–Dordrecht–Rossow area). The
map is modified and compiled from Gevers (1928), Dingle et al. (1983), Keyser (1997) and Woodford et al. (2001). It should be noted that the symbols
for the breccia pipes represent pipe clusters and not individual pipes. (b) Simplified geological map of the study area showing the spatial relationship
between hydrothermal vent complexes and sill intrusions in the Dordrecht–Rossow area. The map is based on unpublished data from the Council of
Geoscience (L. Chevallier, pers. comm.). (c) Simplified stratigraphic overview of the Karoo deposits in South Africa. The dominant sediment types are
sandstone (Ss) and shale (Shl). The stratigraphic positions of sill intrusions and hydrothermal vent complexes (HTVC) are shown for reference. The
stratigraphy is based on a compilation by Catuneanu et al. (1998), and the position of sill intrusions is from Chevallier & Woodford (1999). The
thicknesses of the individual groups are not to scale. The given thicknesses of the formations below the Stormberg Group are from borehole WE1/66
(Leith & Trümpelmann 1967), whereas a thickness of c. 700 m for the Stormberg Group in the same area is adopted from Dingle et al. (1983). The
thickness of the Karoo Basin sediments varies considerably within the basin (e.g. Rowsell & De Swardt 1976), but for this study the WE1/66 stratigraphy
is considered representative, as the borehole is located just north of our study area (308539500S, 268509260E). A total of 19 sill intrusions are present in the
borehole (a total of 956 m).
Fig. 3. Rocks and textures from
hydrothermal vent complexes. (a)
Photograph showing the main lithologies at
the Witkop III hydrothermal vent complex.
(b) Photomicrograph of sandstone (Unit I)
from the inner zone of Witkop III showing
quartz and feldspar grains without any
altered igneous material. (c) Sandstone with
nodules of zeolite cement from the outer
zone at Witkop III. This rock type is
present in about 30 m of stratigraphy, and is
called Unit II. (d) SEM backscatter image
of a sample from Unit II showing
authigenic zeolite cement in sandstone. The
zeolite fills a fracture in K-feldspar,
documenting an early stage of precipitation.
Also noteworthy is the absence of
authigenic K-feldspar and quartz cement.
(e) Close-up of a sediment breccia from
Witkop II with an angular claystone
fragment in a matrix of fine-grained
sediment breccia. (f) Photomicrograph of a
sediment breccia from a pipe at Witkop III.
The dark fragments represent claystone, and
are aligned parallel to the wall of the pipe.
The matrix is fine-grained sandstone.
674
H. SVENSEN ET AL.
resembles the Clarens Formation sandstone in texture, colour and
grain size, although it may locally be more clast-rich. The
cream-coloured sandstone is locally interbedded with breccia
(Unit III), and contains varying degrees of sedimentary rock
fragments. The topographic high in the inner zone of Witkop III
is dominated by sandstone, and has a characteristic reddish
weathering crust showing secondary silica cement and feldspar
dissolution. The weathering has resulted in problems identifying
the primary mineral assemblages in the sandstone. Clasts of other
sedimentary rocks (shale, siltstone) occur occasionally.
Zeolite-cemented sandstone (Unit II)
The basal parts of the Clarens Formation in the northern parts of
Witkop III contain distinct white nodules of zeolite (Fig. 3c).
The nodules are commonly circular and about 1 cm in diameter,
and represent sandstone cemented by authigenic laumontite.
Sandstone with this nodular type of zeolite cement (Fig. 3d) has
not been observed elsewhere in the study area, although the
Clarens sandstone immediately underlying the lavas contains a
zone of a few metres thickness with zeolite nodules. The zeolite
sandstone unit contains thin (,20 cm) horizons of siltstone
without zeolite cement. The thickness of the zeolite-cemented
unit is about 30 m, and features such as sandstone dykes and up
to 30 cm high calcite-cemented cylindrical pipe structures are
abundant. The pipes are interpreted as dewatering structures.
Sediment breccia (Unit III)
The sediment breccia comprises clasts of sedimentary rocks
(sandstone, siltstone and claystone) within a matrix of finegrained sandstone (Fig. 3e and f). The sediment breccia is matrix
supported with a greenish colour, in contrast to the creamcoloured Clarens sandstone and vent complex sandstone. The
modal content of clasts varies considerably, from high (.50%)
to sporadic (,5%), and the clasts vary greatly in size, from
millimetre to decimetre scale. The matrix consists of sandstone–
siltstone and fine-grained breccia with clasts ,2 mm. Likewise,
the shapes of the clasts are often both angular and subrounded
within the same exposure. Many of the sandstone clasts resemble
the Clarens sandstone in colour and grain size, whereas others
are identified as Elliot and Molteno sandstone based on colour,
mineralogy and grain size. Molteno sandstone is easily identified
by its coarse grain size (coarse sand) and abundant visible quartz
cement (see Turner 1972), whereas the Clarens sandstone is
identified by its small grain size (fine sand) and cream colour.
Elliot sandstone outside the vent complexes has commonly a
characteristic red to purple colour, but red clasts are rare in the
breccias. It should be pointed out that it is difficult to trace most
of the sandstone and siltstone clasts to their source formations.
However, the Elliot Formation is regarded as the major source of
siltstone fragments as siltstone is more abundant in the Elliot
than in the Molteno and Clarens formations.
Clasts and boulders of rounded sandstone are abundant in
Witkop II and III. They are well cemented, with secondary porefilling calcite. Aggradation is a possible explanation for the
rounded and smooth surfaces of the boulders, and could possibly
have occurred during transport in the vent complex. Furthermore,
micritic calcite clasts (up to 5 cm in diameter) with a small
component of detrital minerals occur locally in the sediment
breccia. The micritic calcite clasts may represent remobilized
pedogenic carbonate, which is common locally in the Clarens
and Elliot formations (see Eriksson 1981).
No igneous boulders or fragments were found in the breccia at
Witkop III. At Witkop II, however, a basaltic boulder (25 cm in
diameter) is present in sediment breccia near the top of the vent
complex (Fig. 4). The boulder comprises tabular crystals of
plagioclase in a fine-grained matrix, suggesting relatively rapid
cooling. It has previously been suggested that the sediments from
the inner zone of Witkop III contain altered glass shards but no
juvenile igneous material (Seme 1997). We have not been able to
demonstrate the presence of volcanic material or altered igneous
material in the vent sandstone or in the sediment breccia,
although identification could be difficult due to alteration to clay
minerals. The clay-dominated fragments in the studied sediment
breccia, which potentially could represent altered mafic material,
are of clastic origin (see Fig. 3f). The zeolite in sandstone could
have formed in situ from reaction between water and mafic
material. However, we have not been able to positively verify the
presence of altered volcanic material texturally associated with
the zeolite (by using optical and electronic microscopes).
Furthermore, the zeolite in Unit II is pore filling (c. 10–20
vol.%) in 30 m of Clarens stratigraphy, suggesting a major
precipitation event.
The Witkop III hydrothermal vent complex
The sediment-dominated Witkop III hydrothermal vent complex
is situated NW of Dordrecht in the Eastern Cape Province (Fig.
2b). In this area, the Clarens Formation is about 140 m thick,
including two main horizons with dune structures. Volcaniclastic
sediments are present in the upper parts, overlain by fine-grained
laminated sandstone close to the base of the Drakensberg Group
flood basalts.
External structure
The base of the Witkop III complex crops out at the transition
between the Elliot and Clarens formations. The complex overlies
a sill intrusion cropping out in the Molteno Formation (Fig. 2b).
The complex is divided into an inner and outer zone. The
diameter of the complex is about 700 m with the inner zone
having a diameter of 400 m. The outer zone comprises structurally modified Clarens strata with dips ranging from background
values (c. 58) to about 608 into the complex (Figs 5 and 6). The
inner zone is a topographic high, rising about 200 m above the
grass-covered plains in front. The boundary between the inner
and the outer zone is exposed in a gully with slumped sediments
(Fig. 6a).
Facies assemblages
Sediment breccia (Unit III) overlies tilted Clarens sandstone in the
western parts of the inner zone, and is overlain by Unit I sandstone
containing few to no clasts (Fig. 6b). Sedimentary structures in
both units are sparse, but channel-like strata of conglomerate are
present in sandstone towards the top of log B in Figure 6b. Both
the breccia and the sandstone dip inward towards the centre of the
complex, with dips between 5 and 588. At the border zone
between the inner and outer zone, tilted layers of alternating
sandstone and conglomerate are aligned in contact with sandstone
from the outer zone. Slumping structures in sandstone indicates
soft sediment deformation. The dips of the strata in log B are
consistent throughout the sequence. This can be related to the
tilting of the Clarens sandstone in the outer zone. The basal Unit
III sediment breccia from log B has a massive appearance and a
general absence of sedimentary structures. In the upper half of log
B, Clarens sandstone and reworked breccia dominate. The hetero-
H Y D ROT H E R M A L V E N T S I N T H E K A RO O BA S I N
geneity of the complex is, however, demonstrated in log C from
the eastern side of the complex. Here, the base of the inner zone
comprises a mixture of sediment breccia and sandstone, the
middle part is dominated by coarse sediment breccia (fragments
up to 40 cm), and the upper 15 m is sandstone dominated (Unit I).
However, as a result of weathering of the sediments and poor
exposures, the details in the stratigraphy of the sediments between
logs B and C remain uncertain. Unit II zeolite-cemented sandstone
is present north of the outer zone.
Logging of chips from the borehole at Witkop III (Fig. 6b)
shows that only the upper 2 m comprise sediment breccia. The
rest of the borehole penetrates Clarens and Elliot strata. The
Elliot–Clarens transition is located about 50 m deeper within the
borehole than outside the complex, demonstrating the structural
modifications of the sediments surrounding the inner zone.
Furthermore, the dolerite dyke or sill cropping out in the inner
and outer zone of the complex (Fig. 6a) was intersected at 117–
123 m in the borehole.
Sediment pipes and dykes
Circular and elliptical vertical to subvertical sediment pipes are
numerous within the inner zone of Witkop III (Fig. 6a). A total
of 21 large pipes (.0.5 m) and about 40 small pipes (,0.5 m)
have been identified. Small pipes are particularly abundant in
two areas within the complex, where they form topographically
positive structures up to about 30 cm in diameter. The pipes are
all composed of sandstone of Unit I type. The large pipes are
positive structures with sharp contacts with surrounding rocks,
often displaying deformation grooves, grain-size reduction, and
flow banding around the margins (Fig. 7). Many of the large
pipes have diameters of 8–10 m, some up to 30 m. All the large
pipes cut the sediment breccia within the inner zone of the vent
complex, and are filled with both sandstone and sediment
breccia. Detailed mapping of one of the large sandstone pipes
shows an outer partially undulating deformation zone up to
50 cm in thickness (Fig. 7a and b) with both horizontal and
vertical deformation grooves. One sandstone pipe has a sandstone dyke extending about 4 m outward from its wall.
Sediment dykes are abundant within the inner zone (Fig. 6a);
most of them are ,30 cm wide and ,10 m long, pinching out at
both ends. They are easily distinguished from pipes. A few
sediment dykes reach 0.5 m in width. The dykes are composed
of sandstone or sediment breccias, and many are distributed
around the topographic high of the inner zone. Sandstone dykes
are also abundant in the outer zone, usually being a few metres
long and up to 10 cm wide. They are especially common in the
area with zeolite-cemented sandstone. The hinge-point of the
outer zone of the complex (Fig. 5a) is furthermore defined by the
presence of a 0.5 m wide sandstone dyke that cuts at least 30 m
of stratigraphy.
Petrography and hydrothermal minerals
The authigenic minerals in sandstone and breccia from the
Witkop III hydrothermal vent complexes are K-feldspar, albite,
quartz and zeolite. Most samples have clays (mainly illite) as the
dominant matrix minerals, but calcite cement occurs locally. The
K-feldspar is often zoned as a result of substitution of ammonium for potassium (buddingtonite). Zeolite forms late-stage
cement in (1) Clarens sandstone from the outer zone (Fig. 3d),
(2) sandstone from the upper 100 m in the borehole, and (3)
sandstone clasts from sediment breccias (both in pipes and
elsewhere). It should be noted that zeolite is locally present in
675
the Clarens Formation surrounding the Witkop III complex, but
the macro-textures are different (less pronounced nodular
development) from those in Unit II. In the upper part of the
Clarens Formation, zeolite is associated with local alteration of
fine-grained volcanic material (Fig. 8a). Zeolite has also been
identified in sandstone without igneous material, where zeolite
apparently formed during dissolution and replacement of feldspar
(Fig. 8b).
The Witkop II hydrothermal vent complex
The Witkop II hydrothermal vent complex is located 5–6 km
SW of Witkop III (Fig. 2b). The structure consists of two
connected circular structures piercing the Elliot Formation (Fig.
9a). The border between the two circular parts is seen in the field
as a depression in the terrain comprising sediment breccias. The
complex is cut by a dolerite dyke that transforms into a sill by
following the internal structures in the complex, in a similar way
to the dolerite dyke at Witkop III.
Two sediment facies units are identified in Witkop II; vent
sandstone (Unit I) and sediment breccia (Unit III). The bulk of
the complex is composed of inward dipping Clarens sandstone
with dune structures and parallel-bedded horizons overlain by
interlayered sequences of sediment breccia and Clarens sandstone. An important feature of Witkop II is that sediment breccia
cuts both the Elliot Formation and the tilted Clarens sandstone.
These relationships are well exposed in the area between the two
circular structures. In addition, an undulating network of sandstone dykes up to about 1 m thick cuts the breccia. A log from
the southwestern part of the complex shows that a tilted block of
Clarens sandstone is overlain by interlayered sequences of
sediment breccia and sandstone (Fig. 9b). Only one large
sandstone pipe was found at Witkop II, located along the border
of the complex. The pipe cuts the surrounding sediment breccia
with an internal deformation zone along the contact.
Geological synthesis
The Witkop II and Witkop III hydrothermal vent complexes
share many characteristic features: (1) both contain inward
dipping sequences of Clarens sandstone cut by sediment breccia;
(2) both contain sediment dykes and pipes; (3) no, or little,
igneous material is found. Witkop III is considered the more
interesting of the two because of the presence of a wider range
of structures such as pipes and dykes that give information about
the evolution of the complex. However, the best examples of
intrusive sediment breccias come from Witkop II. Zeolite-cemented rocks have so far only been found at Witkop III and
igneous clasts only at Witkop II.
Figure 10 shows an interpretative cross-section through
Witkop III. The boundaries between the inner and outer zones
are well defined from field geology and aerial photographs, but
the structure of the deeper parts of the inferred conduit zone
are poorly constrained because of lack of outcrops. Rock
fragments in breccias and pipes originate from the Elliot and
Molteno formations, which require a diatreme-like structure at
depth. The absence of breccia in the borehole, except in the
upper metres, suggests that the conduit zone is narrow. Tilting
of the surrounding Clarens sediments occurred after the deposition of Unit III breccia. The vent sandstone (Unit I) represents
Clarens Formation sandstone covering the sediment breccias,
possible with a component of sand vented from the sediment
pipes.
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H. SVENSEN ET AL.
Discussion
(Svensen et al. 2004; Planke et al. 2005). The .700 hydrothermal vent complexes mapped in these basins are spatially
associated with deeper sill intrusions. The sills intruded the
shale-dominated basins during the break-up of the North Atlan-
Volcanic basins and hydrothermal vent complexes
Recent studies have shown that hydrothermal vent complexes are
abundant in the Vøring and Møre basins of offshore Norway
Fig. 5. (a) The Witkop III hydrothermal vent complex comprises an outer
zone with structurally modified Clarens strata, and an inner zone with
abundant sediment breccia. The whole Clarens Formation is exposed in
the area, and the Drakensberg Group basalts are seen in the background.
Car for scale. (b) Inward dipping strata of Clarens sandstone at the Witkop
II hydrothermal vent complex. The grass-covered areas on the right
represent sediment breccia and those to the left the Elliot Formation.
N
2030
Limit of
outer zone
Log A
Sandstone, cream colored
Sandstone/siltstone, red/gray
Sandstone, zeolite cemented
Laminated
unit
2010
Dolerite sill
Basalt
Volcaniclastics
155/30 Strike/dip of sedimentary
1990
**
*¤
*
1908 m
Log B
S 31 12' 09.2’'
E 27 13' 47.7’'
155/30
100/75 125/66
128/12
***
142/56
X
1956 m
*
¤¤*
¤*
¤
*
*
202/18
Clarens Fm.
*
¤
82/05
60/22
60/20
45/27
1870
1850
1830
Sandstone pipe
Dolerite dyke
Sediment breccia pipe
Sandstone dyke
Area with small
sandstone pipes
Breccia dyke
Slumped
sediments
1790
Borehole
1608
Base
vent fill
Borehole
1890
1810
Sediment breccia
(Unit III)
Zeolite cemented
sandstone (Unit II)
Vent complex
sandstone (Unit I)
155/30
Dune Log A
unit
100 m
Elliot Fm.
128/12
112/80
124/58
148/12
1910
Log C
Limit of
outer zone
Log C
Log B
Tilted
Clarens
105/60
Limit of
inner zone
Inner zone
Dune
unit
1930
Elliot Fm.
*
034/48
Meters above sea level
1950
060/40
Clarens Fm.
075/60
structures
Massive
unit
060/60
1970
B
Sediment breccia
Background
Sediment pipe level
2050
Cover
Tilted
Clarens
Undifferentiated
A
90/07
Drakensberg Gr.
Fig. 4. (a) Crystalline basaltic boulder from an outcrop at the Witkop II
hydrothermal vent complex. (b) SEM backscatter image of the basaltic
boulder. The dominant minerals are plagioclase (Plag) and pyroxene
(Pyr). The tabular quench-like plagioclase crystals in the relatively finegrained matrix suggest rapid cooling of the parent melt.
Fig. 6. (a) Geological map of the Witkop
III hydrothermal vent complex. Three
composite logs based on surface exposures
are presented in (b). A prominent feature of
this locality is the presence of wellpreserved sediment pipes reaching tens of
metres in diameter. (b) Stratigraphy of the
Witkop III area, showing sedimentary strata
from the Elliot and Clarens formations
unaffected by the vent complex formation
(left), surface logs of breccias and
sandstone (logs A–C), and a log of the
borehole. The ‘background log’ is
constructed based on exposures between the
Elliot Formation (exposed underneath Unit
II) and the Drakensberg Group lavas east of
the complex.
H Y D ROT H E R M A L V E N T S I N T H E K A RO O BA S I N
677
Fig. 7. Sediment pipes. (a) Sandstone pipe (6 m 3 6.5 m) cutting sediment breccia in the inner zone of Witkop III. The pipe has a 40 cm thick internal
deformation zone along the margin. Both vertical and horizontal deformation grooves are present within this zone. (b) Detailed map of the sandstone pipe
shown in (a). The partly undulating border zone is typical for the pipes at Witkop III. (c) Close-up of a deformation zone in a sandstone pipe at Witkop
III. The near-vertical deformation grooves and the slate-like appearance of the sandstone should be noted. (d) Thin-section photomicrograph of a
deformation band showing variation in grain size between the matrix and the deformation zone.
A
290/52
Cover
290/45
78/20
*
S 31 15’ 35’’
124/42
114/54
15
0m
Slumping
130/50
100 m
112/50
068/20
200 m
Elliot Fm.
Deformed sediments
Dolerite dyke
Sandstone (FU II)
Striated: Eolian dunes
Sediment breccia
and conglomerate (FU IV)
Sandstone and sediment
breccia (undifferentiated)
Sandstone dyke
*
Sediment pipe
Ero
sio
Clarens Fm.
?
064/22
E 27 09’ 54’’
Facies unit II
10
055/66
D
320/55
Fault
Doleritic boulder
0
Fig. 9. (a) Geological map of the Witkop II hydrothermal vent complex
showing two circular structures defined by inward dipping Clarens
sandstone and sediment breccia. (b) Log from the southeastern part of
the Witkop II hydrothermal vent complex; 15 m of sediment breccias
overlie a tilted block of Clarens sandstone. The strike and dip of the
layers are consistent throughout the section, suggesting tilting of the
whole sequence after breccia deposition.
200 m
Elliot Fm.
300 m
Molteno Fm.
np
rof
ile
5
Zeolite
Hydrothermal alteration
and fracturing
D
330/45
Meters
?
Sediment
pipes
20
228/22
225/35
B
Log
262/42
Borehole
A
N
Sediment breccia
(Facies unit III)
Vent sandstone
(Facies unit I)
Zeolite-cemented
sandstone (Facies
unit II)
Fig. 10. Composite schematic cross-section through the Witkop III
hydrothermal vent complex. The section is based on outcrop data and
borehole information. (See text for explanation.)
678
H. SVENSEN ET AL.
total number of vent complexes in the Karoo Basin would have
been about 3000. Not included are hundreds of sedimentdominated cylindrical pipes present in the Ecca Group (‘breccia
pipes’; Woodford et al. 2001). They possibly represent the deep
roots of hydrothermal vent complexes.
Pressure build-up mechanisms
Fig. 8. SEM backscatter images from Clarens Formation sandstone
surrounding the Witkop III complex. (a) Possible volcanic material
(glass) now altered to chlorite and illite. The texture differs from
anything seen in the sediment breccia from the hydrothermal vent
complexes. (b) Zeolite-cemented Clarens sandstone (from the upper part
of the uppermost dune unit in Fig. 6b). The absence of other authigenic
minerals (indicating early zeolite precipitation), and that the zeolite forms
pseudomorphs after feldspar (albite), should be noted.
tic, at c. 55 Ma. The total number of hydrothermal vent
complexes may be as high as 2000–3000. From the size of the
volcanic basin (85 000 km2 ) this results in an average of 20–40
complexes per square kilometre. The stratigraphic level where
.95% of the hydrothermal vent complexes terminate continues
beneath the extrusive basalts, and no hydrothermal vent complexes have been identified within, or above, the extrusive
sequence (Planke et al. 2005). An important conclusion from
these studies is that the hydrothermal vent complexes are
spatially associated with sill intrusions at depth.
When adopting these results to the Karoo Basin, hydrothermal
vent complexes were probably present where sills crop out today.
As a result of erosion, only a small part of the original size of
the Stormberg Group is present, and thus the original number of
hydrothermal vent complexes was probably higher. Considering
the exposed area of the Stormberg Group (c. 42 000 km2 ) with c.
325 mapped hydrothermal vent complexes (e.g. Du Toit 1954),
the resulting vent complex density is one per 130 km2 . If scaled
to the area with present-day sill intrusions (c. 390 000 km2 ), the
The hydrothermal vent complexes have previously been termed
diatremes and volcanic necks, and have, since the pioneer work
of Du Toit (1904, 1912), been interpreted as the result of phreatic
or phreatomagmatic activity (e.g. Gevers 1928; Botha & Theron
1966; Coetzee 1966; Taylor 1970; Dingle et al. 1983; Seme
1997; Woodford et al. 2001). The Witkop II and III hydrothermal
vent complexes are spatially associated with sill intrusions (Fig.
2b), but a direct relationship between conduit zones and contact
aureoles cannot be demonstrated because of lack of exposures
and boreholes. A general genetic relationship between sills and
vent complexes is, however, supported by interpretations of
seismic data from the Vøring and Møre basins of offshore midNorway, where it is shown that hydrothermal vent complexes are
rooted in aureole segments of sill intrusions (Jamtveit et al.
2004; Planke et al. 2005). The general lack of igneous material
in the hydrothermal vent complexes strongly suggests that they
are rooted in a zone without major magma disintegration. We
have recently proposed a model of vent complex formation by
heating and boiling of pore fluids in contact aureoles around
shallow sills (Jamtveit et al. 2004). In this model, boiling of pore
fluids may occur at depths as great as c. 1 km, and overpressure
and possibly venting occur if the local permeability is low. Thick
sills are common in the Stormberg Group sediments, at least in
the Molteno Formation, which can be assumed to have caused
shallow (,1 km) boiling and expansion of pore fluids in contact
aureoles. A high-permeability host rock requires a very rapid
pressure build-up compared with permeability to initiate hydrofracturing. Following hydrofracturing, the gas phase may expand
and lead to a velocity increase during vertical flow through the
conduit zone. Thus, the vent formation mechanism bears resemblance to shallow breccia-forming processes in hydrothermal and
volcanic systems (e.g. Grapes et al. 1973; Navikov & Slobodskoy
1979; Lorenz 1985).
In systems dominated by fragmentation of magma (e.g.
kimberlite pipes and diatremes), the resulting conduit zone
will comprise mixtures of sediments and igneous material,
and associated surface deposits dominated by pyroclastic
material (e.g. Navikov & Slobodskoy 1979; Lorenz 1985;
Clement & Reid 1989; Webb et al. 2004). Kimberlite pipes
are generally formed from fragmentation of deep dyke
complexes (e.g. Lorenz 1985; Clement & Reid 1989), and
this mechanism may also explain the formation of the
phreatomagmatic complexes in the Karoo Basin (e.g. Surtees
1999; McClintock et al. 2002). The data from Witkop II
and Witkop III support the theory that hydrothermal vent
complexes represent cylindrical conduit zones formed by
overpressured gas and fluids originating in contact aureoles
within sedimentary rocks.
Eruption and deformation
At both Witkop II and Witkop III, sediment breccia cut or
intrude the Elliot and tilted Clarens formations. Fragments up to
50 cm in diameter within breccia imply high transport velocities
during phreatic activity, supported by the presence of craters at
the palaeo-surface at some hydrothermal vent complexes. The
H Y D ROT H E R M A L V E N T S I N T H E K A RO O BA S I N
basalt-dominated Welgesien vent complex, located near the
Brosterlea volcanic complex west of Dordrecht, has a crater filled
with lacustrine deposits directly overlying volcaniclastic rocks
(Marsh & Skilling 1998).
A model involving one (Witkop III) or two (Witkop II) main
eruptive phases is favoured based on, for example, the ring
structures seen on the aerial photographs of Witkop II and the
logs from Witkop III. Mixtures of brecciated and fragmented
sediments from well-consolidated and cemented strata were
transported to the surface during eruptions, and mixed with
loosely consolidated sandstone or sand from higher in the
stratigraphy. Rounded blocks and chaotic structure of parts of the
inner zone of Witkop III can be explained by partial reworking
or aggradation in the conduit zone.
Dipping of sedimentary strata towards vent complexes is
known from mud volcanoes and about 40% of the hydrothermal
vent complexes of offshore Mid-Norway (Planke et al. 2005),
and has been reproduced in analogue experiments (Woolsey et
al. 1975). Furthermore, phreatomagmatic eruptions are commonly associated with collapse and inward tilting of strata (e.g.
Lorenz 1985). At both Witkop II and III, tilted sequences of
breccias overlie massive tilted blocks of Clarens sandstone.
Tilting of both breccia and Clarens sequences shows that
collapse occurred after deposition of the vented material. The
tilting of the outer zone of the Witkop III complex can be
explained by mass transport from either the base of the Clarens
or the underlying formations. The zeolite-cemented outer zone at
Witkop III could have acted as a rigid layer through which
unconsolidated sand could be fluidized. This explains the
sandstone dyke in the hinge-point of the outer zone of Witkop
III.
Formation of sediment pipes and dykes
Sediment pipes and dykes may form from a variety of pressure
build-up mechanisms, ranging from boiling (Jamtveit et al.
2004), overpressured clastic beds with brines or petroleum (e.g.
Lowe 1975; Hannum 1980; Mount 1993; Shoulders & Cartwright
2004), flow of ground water (Guhman & Pederson 1992), and
impact cratering (Kenkmann 2003). Sediment dykes and pipes at
Witkop III emphasize the dynamic nature of the hydrothermal
vent complex. The large sediment pipes at Witkop III postdate
the breccia formation, as documented by cutting relations. Both
brecciation and fluidization (transport of granular material in a
fluid) were probably involved during this stage of pipe formation.
How high was the transport velocity in the sediment pipes?
Minimum fluidization velocities in pipes are commonly of the
order of several hundred metres per second during phreatic
explosive events (e.g. Navikov & Slobodskoy 1979; Zimanowski
et al. 1991; Kurszlaukis et al. 1998; Boorman et al. 2003).
However, minimum transport velocities required to form sand
pipes may be as low as 1 cm s1 (see Lowe 1975). Rapid
formation of the large pipes, and hence a relatively high velocity,
is supported by wall deformation structures (lineation, fracturing,
grain-size reduction; see Fig. 7), indicating high internal pressure. As a consequence, the pipes acted as conduits for sediments
and fluids that were rapidly transported to the surface. This
model implies that the sediment pipes represent conduit zones of
sediment volcanoes.
Hydrothermal activity
Local occurrence of zeolites within the inner and outer zones of
the Witkop III hydrothermal vent complex represents an anomaly
679
compared with the normal Stormberg Group mineralogy, and
suggests a hydrothermal influence. Zeolites have previously been
found only in Clarens sandstone a few metres below the
Drakensberg Group basalt (Potgieter et al. 1982). Considering
the geological setting of Witkop III, the zeolite documents a
precipitation event related to the formation of the hydrothermal
vent complex. The zeolite in Witkop III is laumontite (Table 1
and verified by XRD), which can form in a range of environments, during both diagenesis and hydrothermal alteration. However, precipitation requires alkaline (with high Ca/Na activity
ratio) and silica-rich fluids (e.g. Chipera & Apps 2001; Iijima
2001). In the Witkop III system, the source of alkalis for
precipitating zeolite could either be albitization of plagioclase or
alteration of volcanic material (see Boles & Coombs 1977; Iijima
2001). The general absence of volcaniclastic horizons or igneous
clasts in the Witkop III complex makes the possibility that the
zeolites in Unit II were formed by in situ alteration unlikely. The
absence of typical hydrothermal mineralization (i.e. silica varieties, sulphides, epidote, amphibole) suggests that the hydrothermal event was short lived, and that the near-surface parts
experienced relatively low temperatures.
Timing
The present-day exposures of the Witkop III vent complex
terminates about 65 m below the base of the Drakensberg Group.
This shows that the phreatic activity predated the lava flows in
this area. Additional evidence supports phreatic activity prior to
flood basalt eruptions: (1) the vent complexes rarely cut the
extrusive basalts, although some phreatomagmatic complexes
postdate the earliest flood basalts (Stockley 1947; I. Skilling,
pers. comm.); (2) as at Witkop III, blocks of extrusive basalt are
not found within other sediment-dominated vent complexes (Du
Toit 1912; Gevers 1928; Seme 1997); (3) pyroclastic horizons
are located within the upper parts of the Clarens Formation
sandstone (e.g. Botha & Theron 1966; Lock et al. 1974; Dingle
et al. 1983; McClintock et al. 2002). A supportive conclusion is
that phreatomagmatic eruptions also predated the main phase of
the time-equivalent Jurassic Ferrar extrusive volcanism in Antarctica (Hanson & Elliot 1996; White & McClintock 2001). We
conclude that the phreatic volcanism in the Karoo Basin was
synchronous with sill emplacement, slightly predating the main
phase of extrusive activity.
Formation model
Based upon observations and interpretations from Witkop II and
Witkop III, we propose a qualitative model for the formation and
evolution of sediment-dominated hydrothermal vent complexes
(Fig. 11). The processes leading to the formation of hydrothermal
vent complexes are initiated during sill emplacement (Fig. 11a).
Boiling of pore fluids and metamorphic reactions in contact
aureoles may increase the fluid pressure and initiate hydrofracturing. Hydrofracturing in aureoles causes vertical fluid migration
and results in extended brecciation of sedimentary rocks. A
conduit zone to the surface is ultimately produced (Fig. 11b),
and some of the eruptive driving force may be the result of fluid
expansion during decompression in the conduit zone. The
conduit zone is cylindrical, becoming cone-shaped closer to the
surface because of increasing velocities and erosion of wall rock.
The igneous component in the conduit zone is negligible,
indicating insignificant hydraulic brecciation of the sill. Circulation of hydrothermal solutions causes zeolite precipitation at
various depths in the system, in both the inner and outer zones of
680
H. SVENSEN ET AL.
Sill emplacement & aureole
Table 1. Microprobe analysis of laumontite from Witkop III
Sample:
Type:
Latitude (S):
Longitude (E):
Pipe formation & eruption
K01HS-44
K02HS-28
K03HS-20
Clarens sandstone Sediment pipe Sediment breccia
31811957.70
31812911.50
31812908.50
27813952.90
27813944.90
27813942.10
0.28
Na2 O
20.23
Al2 O3
51.98
SiO2
0.82
K2 O
CaO
11.09
Total
84.40
Structural formula, based on 48 oxygen
Na
0.17
Al
7.52
Si
16.40
K
0.16
Ca
3.75
Total
28.01
Ca þ Na þ K
4.08
Al þ Si
23.92
0.08
19.86
52.98
1.54
10.41
84.87
0.15
19.98
53.12
1.60
10.36
85.21
0.05
7.35
16.64
0.31
3.50
27.86
3.86
24.00
0.09
7.37
16.63
0.32
3.47
27.89
3.89
24.00
Fluidization
Brecciation
Conclusions
The structure and sediment petrography of two hydrothermal
vent complexes in the Karoo Basin give insight into the
dynamics of phreatic activity in volcanic basins. The complexes
represent sediment-dominated piercement structures in the basin.
The following conclusions may be reached.
(1) The formation of the hydrothermal vent complexes was
synchronous with sill emplacement, slightly predating the major
phase of Karoo flood volcanism.
(2) Phreatic explosive activity formed sediment breccias within
the conduit zone, with rock fragments from different stratigraphic levels.
(3) Ejected material (sediment breccia and sand) fills the vent.
(4) Hydrothermal fluids affect sediments in shallow and
intermediate parts of the complex and precipitate zeolite.
(5) Fluidization of sand and breccia occurs in pipes during
further cooling of the underlying sill complex, and documents a
late phase of pressure build-up. The pipes terminated at the
surface, resulting in sediment volcanism.
This study was supported by grant 146832/431 from the Norwegian
Research Council. Continuous support by VBPR, TGS-NOPEC, and the
industrial participants in the ‘Petroleum Implications of Sill Intrusions’
project is gratefully acknowledged. We thank G. Marsh for discussions
and valuable support during our trips to South Africa, the Department of
Water Affairs and the Council for Geoscience (South Africa) for borehole
drilling and logistics, S. and N. Polteau, S. A. L. Sali, C. Haave and D.
Liss, for the company and assistance during fieldwork, and D. Cole for
sharing his knowledge on the Karoo Basin. We would also like to thank
Zeolites (Z)
Fracturing
Sill+aureole
B
A
1 km
Tilting & collapse
Z
Sediment
dykes
the complex. Formation of the conduit zone leads to wall-rock
collapse and tilting of surrounding sedimentary strata (Fig. 11c).
The collapse initiates fluidization of sediments through pipes and
dykes. Material removed from the wall rock or the conduit may
be fluidized through pipes and erupted at the surface through
small sediment volcanoes (Fig. 11d). Erupted material from the
sediment pipes mixes with surface sediments. The surface
topography can be a depression or a crater that fills at rates
depending upon the local sedimentation regime.
Z
Sediments
0.5 km
Late stage activity
Sed. Volc.
Z
Z
Z
Z
Hydrothermal
fluids
C
D
Fig. 11. A schematic model for the formation and evolution of sedimentdominated hydrothermal vent complexes. The model represents a
synthesis of field observations and data from the Karoo Basin. (See text
for explanation.) (a) Emplacement of melt and aureole formation. (b)
Hydrofracturing in the source region and conduit formation. (c) Wallrock collapse and tilting. (d) The late-stage activity.
I. Skilling for a constructive review, and M. Erambert for electron
microprobe analytical support.
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Received 8 March 2005; revised typescript accepted 18 November 2005.
Scientific editing by Sally Gibson